American Board of Internal Medicine (ABIM) Certification Practice Exam

The diagnosis of Myasthenia Gravis is based on clinical findings, but what other essential test should be performed?

1. Magnetic resonance imaging

2. Electromyography

3. Chest X-ray

4. Serum PCR

The correct answer is: Electromyography

The diagnosis of Myasthenia Gravis (MG) primarily relies on clinical findings such as muscle weakness that worsens with activity and improves with rest. However, electromyography (EMG) is an essential test that can help confirm the diagnosis. Specifically, repetitive nerve stimulation during EMG can demonstrate a decremental response in muscle action potential with repeated stimulation, which is characteristic of MG. This finding indicates a problem at the neuromuscular junction, which aligns with the underlying pathology of the disease. Other tests, while potentially useful in the diagnostic process or assessing associated conditions (like a chest X-ray for thymoma), do not provide the same direct evidence of the neuromuscular transmission issue that EMG does. Chest X-rays may help identify any associated thymic abnormalities but are not diagnostic for MG itself. Magnetic resonance imaging is typically not relevant in the context of diagnosing MG, and serum PCR would not provide specific information regarding this neuromuscular condition. Therefore, electromyography is the key supplementary test for establishing the presence of Myasthenia Gravis.